This invention relates to heating evaporant material, particularly a method and apparatus for applying radiant heat to such material.
Evaporation heaters for sublimable material have been formed from sheet refractory metal such as tantalum, in the form of a resistively heated box, see for example U.S. Pat. No. 3,598,958 to Davey et al. (1971). The drawback of such heaters is that they are relatively expensive, require considerable electric power and often chemically react with the evaporant being heated causing erosion and failure of the heater.
Another type of evaporation heater employs a coiled tungsten heater mounted within an annular chamber, the evaporant material being placed in an annulus around such heater; e.g., a shown in U.S. Pat. No. 4,002,880 to Davey et al. (1977). This type of heater is employed for relatively large amounts of evaporant or large sized annular evaporant bodies and is not efficient for evaporating smaller sized evaporant including pellets. Further the heater applies heat from but one side of the material, the inside. In addition such heaters have a considerable number of component parts and are of sizable construction.
Evaporation heaters of simpler construction have been proposed such as wire coil heaters, e.g. U.S. Pat. Nos. 2,301,456 to Sabine (1942) and 4,054,500 to Parent (1977). In the former coil type heater patent, the evaporant material, e.g., a pellet, is placed in direct contact with the heater coils which subjects such heater to chemical reaction, erosion and failure. In the latter type of coil, which is coated with alumina to form a crucible, such protective coating delays but does not prevent the resultant erosion and failure of this type of heater.
Accordingly, an evaporation heater which is of uncomplex construction and avoids the chemical reactive and erosion problems discussed above had heretofore not been available, and there is a need and market for such improved heater which substantially overcomes the above prior art shortcomings.